Inductive power couplings are used to transfer electrical power to mobile equipment or to other devices under conditions that do not favor the use of trailing cables, sliding contacts, or the like. Since an inductive coupling transfers power by means of a magnetic field that extends through an air gap, safety, durability, and reliability are improved by the elimination of the problems associated with frayed cables and exposed contacts.
Electric power is used in mobile equipment to operate devices such as battery charges, propulsion equipment, control systems and the like. These devices typically require electrical power from a constant-voltage source. Accordingly, there arose a need for an inductive coupling capable of providing power at a constant voltage.
Constant voltage transformers, also known as ferro-resonant transformers, are an effective source of constant voltage. A linear reactor is used in or with these transformers in conjunction with a capacitor and a saturable steel core in order that the effects of varying input voltage and output current are compensated for in maintaining a relatively constant output voltage. A precise air gap in a magnetic shunt between input and out windings is typically used in these transformers in order to achieve the linear reactance that is required.
An inductive power coupling consists of two inductors that are separated in operation by an imprecise and relatively large air gap. This construction precludes the use of a precisely controlled shunt reactance between the input and output windings in the manner of a ferro-resonant transformer. Accordingly, some other means of voltage regulation must be provided.
It has been proposed to regulate the voltage provided by an inductive coupling by means of switchable taps on the output winding of the coupling, but such an expedient provides only coarse regulation in a series of discrete steps. It has also been proposed to use solid-state switching regulators, but such regulators are costly and electronically complex.
In U.S. Pat. No. 3,914,562 (Bolger), means are proposed for regulating the output voltage from an inductive coupling by varying the air gap between the input and output inductors. In U.S. Pat. No. 4,331,225, also issued to Bolger, means are proposed for regulating the output voltage by switching various ones of a plurality of capacitors across the output winding. However, both of these devices require active controls that suffer from complexity and high cost, and hence, prior to the present invention, the need remained for an uncomplicated and economical means to regulate the output voltage from an inductive coupling.
It is, therefore, a general object of the present invention to provide an improved inductive power coupling for coupling power through an air gap that produces a constant voltage output despite fluctuations in the input power supply and the output load.
Another object of the invention is to provide an inductive power coupling that produces a constant voltage output and is constructed economically with a minimum of components.
Another object of the invention is to provide an inductive power coupling that is constructed so that the core of the pickup or output inductor will operate at its saturation level and, despite variations in supply power to the input inductor and the output load, mutual flux between the two inductors will remain essentially constant, thereby providing a constant output voltage from the pickup inductor.
Still another object of the invention is to provide an improved inductive power coupling that is particularly useful in combination with a battery charging circuit for an electrically powered vehicle.
Yet another object of the invention is to provide an inductive power coupling in combination with a battery charging circuit which automatically operates in a trickle charge mode at certain predetermined levels of battery condition.